The main objective of the proposed research is to elucidate the mechanism of action of a variety of drugs, toxins and other chemicals on nerve membranes. Special emphasis is placed on the mechanisms whereby these drugs affect membrane ionic permeabilities, the key factor for nerve excitation. Voltage clamp techniques are applied to the intact and internally perfused squid giant axons to measure ionic permeabilities under a variety of experimental conditions including altered ionic environment and pH in both external and internal phases of the axon and altered calcium concentrations in external phase. These studies enable us to clarify the mode of action of drugs not only in terms of ionic permeabilities but also in terms of the site of action and active form of the drugs. The drugs to be used include neuroactive toxins such as grayanotoxins and tetrodotoxin, local anesthetics, and protein and sulfhydryl reagents. The drug-receptor interactions are also studied using spin-labeled local anesthetics and spin-labeled acetylcholine with the aid of various techniques including electron spin resonance nuclear magnetic resonance, affinity chromatography, etc. These results will be integrated at a high level to interpret the mechanism of action of drugs at the membrane and molecular level.